The acronym STEM (Science, Technology, Engineering and Mathematics) was formulated by the American National Science Foundation to demarcate the general trend of diminished interest of students for careers in science, technology, engineering, mathematics and related fields, and to encourage these careers among students. In Croatian scientific nomenclature and research tradition, the acronym STEM would indicate the fields of natural and technical sciences as well as some of their interdisciplinary aspects. The STEM problem is a relatively new and socially very relevant research area in Europe, especially in the Republic of Croatia. The issue of young people’s STEM related educational achievement and career aspirations is important because it provokes further research, educational, social and technology related issues and implications. The interest of European youth for these vocations is declining since 1990s. Researchers, national governments and authorities like EU operate with exact figures which clearly demonstrate a shortage of STEM graduates and a decline in the number of STEM experts. This paper aims to further explain some theoretically driven research question. We intend to consider the level of Croatian primary school students’ self-competence beliefs related to STEM school subjects, where beliefs related to mathematics performance are especially important. Self-competence beliefs represent “children's cognitive representations of how good they are at a given activity” (Freiberger, Steinmayr, & Spinath, 2012, p. 518). They encompass information important for one’s identity and information important for one’s performance. Information about one’s efficacy, or academic self-efficacy beliefs (ASE) represent individuals' convictions that they can successfully perform given academic tasks at designated levels (Bandura, 1997 ; Schunk, 1991). They refer to students' perceptions of their ability to master given tasks or develop specific competences (Bong, 2001) or judgments that reflect task-specific performance expectations (Zimmerman, 2000). On the other hand, self-evaluations of specific abilities or qualities are sometimes termed as domain-specific self-concepts, or more broadly as an academic self-concept (ASC) (Marsh 1990, Marsh, Byrne, & Shavelson, 1988 ; Marsh and O' Mara, 2008 ; Schavelson, Hubner, & Stanton, 1976). These constructs have sometimes been used interchangeably despite their conceptual and operational differences (Bong & Skaalvik, 2003). Both facets of self-competence beliefs are important in relation to general academic, mathematical, and verbal domains of school achievement (e.g., Van Dinther, Dochy, & Segers, 2011). The development of the relation between school performance and self-competence beliefs in the STEM field is still not clear, both regarding its structure and causal relations. Several general models developed in the field of school achievement can serve as a starting point in delineating these relations in the STEM field (Byrne, 1984 ; Calsyn & Kenny, 1977 ; Skaalvik & Hagtvet, 1990 ; Valentine & DeBois, 2005): the skill-development model assumes that achievement affects self- competence beliefs ; the self-enhancing model assumes that self-competence beliefs affect achievement ; the reciprocal effects model suggests that achievement and self-competence beliefs affect each other ; and some models assume that external variables affect both achievement and self-competence beliefs. The main objectives of the present paper are to examine the level of self-competence beliefs in STEM school subjects and to explore the relationship between self-competence beliefs and school achievement in STEM school subjects, especially achievement in mathematics. We expected that differences exist in the structure of this relation in groups of students with differing levels of interest for the STEM school subjects. Methods: The data for this paper arise from the JOBSTEM project (www.jobstem.eu), which is a comprehensive longitudinal-sequential (cross- sequential) research project with experimental STEM intervention intended to increase the interest of primary school students for STEM school subjects, help them develop positive attitudes towards STEM, and give them more information about possible future STEM educational and career choices. A total of 1920 primary school students and their parents/guardians from 16 primary schools participate in the study. Students are grouped in three age cohorts, which are followed longitudinally through three measurement waves. Additionally, we also used data from the JOBSTEM pilot study. In that sample there were 360 primary school students (195 boys and 165 girls), attending grades 6 to 8 (age 12 to 15 ; M=13.3 years). The main objective of the pilot- study was to examine some aspects of feasibility of the JOBSTEM research project. As in the operationalization of some of the basic research constructs in the pilot-study the procedure adapted from the UK ASPIRES research project (Archer, et al., 2013) was followed, the data from the pilot-study provide possibilities for some comparisons. We used several research constructs to answer the research questions: (1) Self-competence beliefs related to science, mathematics and STEM school subjects were assessed with a scale consisting of eight items (e.g. “I am just not good at school science”). The internal consistency of the formed composite was good (Cronbach alpha 0.87). Taking into account the content and valence of each item, two subscales were formed as indicators of Positive and Negative STEM self-concepts in STEM school subjects. ; (2) School experiences with STEM school subjects were operationalized through six items (e.g. “I get good marks in science”). The internal consistency of the composite was good (Cronbach alpha 0.81). ; (3) School achievement in STEM school subjects was measured by school marks. In the Croatian school system and school curriculum the STEM area is covered by the following school subjects, depending on the grade: mathematics, nature and society, biology, chemistry, physics, ICT, technical education and geography. Conclusions: Students in general had a fairly positive STEM self-concept. They agreed with the opinion that they will be successful in STEM school subjects if they work hard, felt that they are successful in STEM school subjects, that they don't have problems understanding STEM school lessons. Although the students held positive attitudes, their actual achievement indicated a somewhat different picture. Students are generally less successful in STEM school subjects compared to both their overall GPA, and their marks form non-STEM school subjects. The comparison of school achievement in STEM subjects of students who have a positive STEM self-concept and those who have a negative STEM self-concept yields interesting results. Croatian primary school students with a positive STEM self-concept are significantly more successful in all the school subjects than students with a negative STEM self-concept (ANOVA results: F= 45.8 for Mathematics ; F=28.6 for Biology ; F=44.6 for Chemistry ; F=11.6 for Physics ; F= 12.7 for Computer science ; F=31.9 for Technical culture ; all calculated test are significant at p=0.05). Such an outcome is expected and it is in accordance with the findings that students with well-defined self- competence beliefs in a particular school subject or area also have higher success and show better performance in the corresponding subject. Univariate comparison results indicated that girls are more successful than boys in all the STEM school subjects. This is in line with some our recent findings which show that in primary school girls generally outperform boys (Burusic and Seric, 2015). Multivariate comparisons of boys and girls who hold different STEM self- concepts (positive vs. negative) did not show any statistically significant differences. Neither the effect of gender nor the effect of STEM self-concept was statistically significant, and there were no statistically significant interaction effects of these factors on the school achievement in STEM school subjects.